The accurate diagnosis of faults is an increasingly important aspect of testing integrated circuits, especially in view of ever-increasing gate counts and shrinking feature sizes. For circuits that do not utilize compression techniques during testing, fault diagnosis is relatively straightforward. For circuits that have embedded compression hardware, however, accurate fault diagnosis presents a formidable challenge.
The use of compression during the testing of integrated circuits has become widespread. In general, compression helps reduce the volume of test data required for even traditional stuck-at test sets. Such sets, for example, often exceed the capacity of automatic test equipment (ATE) used to test today's multimillion-gate integrated circuits. Moreover, due to the limited bandwidth between the circuit-under-test (CUT) and the ATE, the use of compressed test data and compressed test responses can help decrease test time, and thus the test cost.
The use of scan-based designs for testing purposes has also become widespread. Scan-based designs provide direct access to the internal nodes of the CUT, and thus can help improve fault diagnosis and silicon debugging. For example, the shallow combinational logic that typically exists between scan cells in scan-based designs can make the diagnosis of many high-performance VLSI devices much easier.
For scan-based designs that also utilize compression hardware, fault diagnosis is typically performed in one of three manners: bypass diagnosis, direct diagnosis, and indirect diagnosis. Of these, indirect diagnosis is typically easier to implement in that it uses simpler hardware, is compatible with existing diagnosis tools, and allows for online diagnosis support. Indirect diagnosis is typically performed in two stages. First, the scan cells of the CUT that are driven by cones of logic affected by actual faults are identified. For example, scan cells that captured unexpected (and thus erroneous) values upon application of one or more test patterns are identified from the compressed test responses output from the compactor. From the scan cells identified from such a procedure (sometimes referred to as “failing scan cells”), one can then apply a second diagnosis technique (for example, using a known diagnosis tool for scan-based designs) that helps locate the physical location of the faulty component or element within the CUT.
Many of the conventional fault diagnosis techniques for CUTs having compaction hardware require substantial additional hardware on the CUT or require multiple test sessions to produce useful results for diagnostic purposes. For these reasons, fault diagnosis has been viewed as impractical for production testing. Accordingly, there is a need for diagnostic techniques, especially indirect diagnostic techniques, that can be performed without substantially increasing the hardware overhead of the CUT and that can efficiently diagnose failing scan cells during production testing.